Hypobromous acid is one of the most potent sanitizers among the oxidizing halogenated compounds. Since it is a weaker acid than hypochlorous acid (pK=8.8 at 25° C.), it is predominant at pH higher than 9.
Alkaline hypobromites can be prepared at low temperature, with good yield, if an excess of hydroxide is provided. A supersaturated solution can be prepared at 10° C., if 90% of the equivalent amount of bromine is fed to a 10 N solution of NaOH or KOH. The pentahydrate NaBrO.5H2O begins to precipitate at −3° C., and keeps precipitating at lower temperatures, while the heptahydrate begins to precipitate at −7° C., and keeps precipitating at lower temperatures. However, the precipitation is slow. This mode of operation enables the preparation of MOBr solution (M=Na, K), but these are not stable enough for practical and commercial application. Concentration increase of the MOBr has a beneficial effect on the stability because of the simultaneous decrease of water concentration
Hypobromites, e.g. sodium hypobromite, can also be prepared from the reaction of bromides with an oxidant, e.g., chlorine or hypochlorite. The reaction with hypochlorite has the disadvantage of yielding equivalent amounts of NaOBr and NaCl. Since the NaOCl solutions themselves contain NaCl in equivalent amount with NaOCl, and mostly contain at most 15.8 wt % NaOCl, the obtainable concentration of NaOBr is relatively low.
The strong oxidizing potential of the hypobromous acid and hypobromites made them very difficult to stabilize. Several classes of stabilizers, among them amides, amines, sulfonamides, melamine, sulfamic acid, cyanuric acid, and hydantoins, have been suggested in the prior art. However, amides and amines are generally oxidized by the hypobromites. Urea is decomposed down to nitrogen and other amides are transformed to amines that in turn can be oxidized to nitrogen.
Sulfamic acid and its salts have been mentioned as stabilizers, being stable to the attack of hypochlorous and hypobromous acids. The latter react at low temperatures with the alkali salts of sulfamic acid, affording chloro- and bromoamidosulfonates XHN—SO3M (X=Halogen). However, some strong oxidants, among them chlorine and bromine, can attack the NH2 function liberating nitrogen.
U.S. Pat. No. 5,683,654 discloses a process which comprises preparing an aqueous solution of unstabilized alkali or alkaline earth metal hypobromite by mixing and reacting the corresponding hypochlorite with a water-soluble bromide ion source and stabilizing the result with an aqueous solution of an alkali metal sulfamate. U.S. Pat. Nos. 5,795,487 and 5,942,126 disclose essentially the same process.
U.S. Pat. No. 6,037,318 describes a process for the manufacture of alkaline bleaching compositions which comprises three steps: a) admixing a source of sodium hypochlorite and an amino compound which may be sulfamic acid, to form a pre-bromine admixture; b) adding to the mixture a source of bromine; and c) adjusting the pH of the resulting mixture to at least 13. However, since hypochlorite solutions generally contain chlorides in an equivalent amount with hypochlorites, the resulting mixtures contain large amounts of sodium chloride. Further, since the stabilizer, e.g., sulfamic acid, and the hypochlorite are mixed before the addition of a bromine source, the efficiency of the stabilizer is decreased, because it reacts with the hypochlorite. The stabilized solution has a low NaOBr concentration because of the low concentration of the starting NaOCl solution.
DE 3398850 discloses stabilizing solutions of sodium hypochlorite with a stabilizer which may be the sodium salt of amidosulfonic acid. It does not teach the preparation of hypobromite solutions.
U.S. Pat. No. 6,068,861 describes a process of making a concentrated liquid biocide formulation, in which bromine chloride and an alkali metal salt of sulfamic acid are mixed. Bromine chloride is difficult to handle and tends to dissociate to bromine and chlorine. It is not a commercial product and must be manufactured by using special skills and expensive installations for keeping it in liquid phase under pressure.
It is a purpose of this invention to provide a method for obtaining stabilized solutions which contain high concentrations of alkali hypobromites.
It is another purpose of the invention to provide such solutions in which bromine is fed as such and not through a more complex source of bromine.
It is a further purpose of this invention to provide such a process that provides hypobromite solutions having an amount of active halogen, expressed as available chlorine, that is higher than that of any solution prepared according to the prior art.
It is a still further purpose of this invention to provide a sanitation method for bodies of water, such as industrial water in cooling towers, pulp and paper waste and the like, in the pH range of 5-10, by feeding the hypobromite solution prepared according to the process of the invention, so that the proper active HOBr concentration (expressed as available chlorine) is achieved.
Other purposes and advantages of the invention will appear as the description proceeds.